Recently, electric vehicles, including a hybrid car that runs on a combination of electric power stored in a secondary cell and fuel, such as gasoline, as a power source, and a secondary cell vehicle that runs on electric power stored in a secondary battery as a power source, have been put into practical use. These electric vehicles include an electric vehicle (for example, a plug-in hybrid car) whose battery is charged with electric power supplied from a commercial power source at a private home. For charging the battery, an electric vehicle charging cable is used in order to connect a socket outlet of the commercial power source and a connector of the electric vehicle.
As this type of battery, there is included a battery requiring a high voltage of about, e.g., 200 to 300V. Also, there is included a battery using a high voltage of 200V as commercial power for charging. Therefore, there arises a safety concern for an electric vehicle or an electric vehicle charging cable.
Further, at an ordinary home, an electric vehicle is usually parked in an outdoor parking space, a car port installed outdoors, or the like. When the battery of the electric vehicle is charged, a charging cable for the electric vehicle is plugged in a waterproof power socket of a fall-out preventing structure provided on an outside wall of a house and is supplied with commercial power from the waterproof power socket to thereby charge the battery. Further, when the electric vehicle is not charged, for example, when the electric vehicle is driven out, it can be considered that the electric vehicle charging cable is accommodated indoors or inside the electric vehicle, e.g., in the trunk. That is, the electric vehicle charging cable is plugged into or pulled out of the waterproof power socket whenever it is used.
In addition, the fall-out prevention type of the waterproof power socket provided on the outside wall of the house has been designed under the assumption that a plug would be kept plugged in the socket for a long period of time. Therefore, if the plug is frequently pulled out and inserted, abnormal heating may be generated due to an imperfect connection. Also, there is a possibility that abnormal heating will be induced by a tracking phenomenon due to high humidity and large amounts of particles and dust outdoors accumulated.
In order to address the above, there has been conventionally suggested a power feed coupler which detects an increase in temperature during the time of charging of a battery and stops the power feeding once an abnormal temperature rise is detected to thereby prevent abnormal heating (see, e.g., Patent Document 1). The power feed coupler disclosed in the above document is provided with a thermistor serving as a temperature detection means in a case having a primary coil therein. When the temperature of the power feed coupler exceeds a set temperature, a temperature abnormality is detected by an increase in resistance of the thermistor such that the power feeding is stopped.
In addition to a bad connection or abnormal heating induced by the tracking phenomenon, there is a possibility that, if an electric leakage occurs during the time of charging a battery by using a high voltage as mentioned above, an excessive current flows and stress is put on the battery or the charging circuit. Due to this, there is a need to avoid abnormal heating or electric leakage by a combination of multiple means, e.g., by providing the power feed coupler of Patent Document 1 and an electric leakage breaker in the power feeding path of the commercial power source. This results in high cost of the equipment and much time for installation.
FIG. 16A shows an example of the charging cable unit AA. A cable connector G provided at one end of a cable 6 is connected to a connector of an electric vehicle, and a plug 3′ provided at the other end of the cable 6 is inserted and connected to the socket BB of a commercial power source, so that a commercial power is supplied to the electric vehicle C to charge a secondary cell installed in the electric vehicle C. FIG. 16B shows the charging cable unit AA′ provided with an electric leakage breaker 5′. When detecting an electric leakage, the earth leakage breaker 5′ cuts off the power feeding to the electric vehicle C (see, e.g., Patent Document 2).
With the charging cable unit AA′ disclosed in Patent Document 2, an electric power can be supplied to charge the secondary cell of the electric vehicle C. However, an amount of power consumed for charging the electric vehicle C or the electricity cost cannot be checked and further an elapsed charging time, charging completion time, or the like cannot be displayed. Accordingly, a user has to directly check the charging cable unit to know whether charging is complete or not, thereby making it inconvenient.
Conventionally, there have been proposed charging systems that can display charging information relating to a secondary cell installed in an electric vehicle or an emergency battery. FIG. 17 depicts one of the charging systems wherein the display function is provided in the electric vehicle C or in the emergency battery D. With this system, the user can find out the charging status of the secondary cell when he is near the electric vehicle C or the emergency battery D, but cannot when he is at a location (e.g., at home) away from a charging place.
In order to address the above drawbacks, there is proposed a charging system which is configured to keep track of the charging status of the secondary cell even when the user is at a location away from a charging place (see, e.g., Patent Document 3). With this system, wireless communications are performed between a transmitter provided at an electric vehicle and a receiver provided at a display device. Thus, charging information of the secondary cell is transmitted from the transmitter of the electric vehicle to the display device that can display the charging information received by the receiver.
With the charging system disclosed in Patent Document 3, the user can check the charging status even if he is at home by having the display device installed at home. However, when it is intended to apply this system to an emergency battery for example, this system can be applied only to an emergency battery equipped with a transmission function because the transmitter is installed at the electric vehicle in the charging system disclosed in Patent Document 3, thereby imposing a limitation to a wide application.    Patent Document 1: Japanese Patent Laid-Open Application No. 2001-10518    Patent Document 2: Japanese Patent Laid-Open Application No. H 5-276674 (Paragraph Nos. [0012] to [0015] and FIG. 1)    Patent Document 3: Japanese Patent Laid-Open Application No. H 5-211724 (Paragraph Nos. [0009] to [0013] and FIG. 1)